1. Field of the Invention
The present invention relates to a fabric for an uncoated air bag as one of safety devices for a vehicle. More particularly, the invention provides a compact, economical uncoated air bag fabric having low air-permeability while holding necessary mechanical characteristics. The invention also provides a compact air bag fabric of which air-permeability is low but increases at high pressure (40 kpa or higher).
2. Description of the Related Art
In recent years, an air bag of which installation rate has been rapidly increased as one of safety devices of a vehicle protects the driver or occupant in the following manner. When a vehicle crashes, the impact is sensed by a sensor, high-temperature and high-pressure gas is generated from an inflator, and the air bag is rapidly developed by the gas to thereby prevent the driver or occupant's body from being jumped out by the impact so that especially his/her head does not collides with the driving wheel, front glass, door glass, and the like.
Hitherto, a base fabric coated with a synthetic rubber such as polychloroprene, chlorosulfonated polyolefine, or silicone rubber is used for an air bag since it has high heat resistance, high air shutoff performance (low permeability) and high non flammability.
The coated base fabrics are, however, heavy and do not have satisfactory flexibility. The manufacturing cost is high and the base fabrics are not recyclable. Therefore, the base fabrics have such drawbacks when each of them is used as a base fabric for an air bag.
Although the drawbacks have been largely improved in a silicone rubber coated base fabric which is still presently used, the fabric is not yet satisfactory.
Recently, an uncoated air bag using an uncoated base fabric for an uncoated air bag is mainstream and the following techniques have been proposed for lighter weight, more preferable foldability and lower permeability.
(1) Method of obtaining light base fabric of low air permeability by performing a shrinking process and a calender process after very compact fabric is woven (Japanese Unexamined Patent Publication No. 1-122752).
(2) Method of obtaining a light base fabric of low permeability of 0.5 cc/cm2/s or lower at differential pressure of 124 Pa by performing a calender procession both sides of the base fabric (Japanese Unexamined Patent Publication No. 4-2835).
(3) Method of obtaining a base fabric of low permeability by performing a chemical shrinking process on the fabric to thereby swell yarn constructing the fabric (Japanese Unexamined Patent Publication No. 6-41844).
(4) Method of mixing a thermoplastic synthetic fiber A having single yarn size of 1.5 d to 7.0 d and a thermoplastic synthetic fiber B having single yarn size of 0.2 d to 1.5 d (Japanese Unexamined Patent Publication No. 8-325888).
As a vehicle is becoming lighter and more compact, an uncoated air bag is requested to be lighter and more compact. Manufacture of a compact, economical air bag fabric of low permeability and light METSUKE (mass) cannot be achieved by the conventional methods (1) to (4) for reasons such that even when the permeability is low under a low differential pressure condition, the pore size fluctuates at high differential pressure and, as a result, the permeability increases.
It is therefore an object of the invention to provide a light mass (METSUKE), compact, economical air bag fabric of low permeability.
Since an air bag has to rapidly develop at the time of a collision to protect the driver or an occupant, an uncoated air bag fabric is requested to have low permeability. In the latter period of development, it is demanded that the permeability increases in order to prevent abrasion on the face which occurs at the time of development.
A fabric satisfying such performance, however, cannot be obtained by the conventional techniques (1) to (4).
It is another object of the invention to provide an economical uncoated air bag fabric having a characteristic such that its permeability is low but becomes high at high differential pressure (40 kPa or higher) in the latter period of development.